RFC 3218 Preventing the Million Message Attack on CMS January 2002
attacker could convince the receiver to act as a particular kind of
oracle. (An oracle is a program which answers queries based on
information unavailable to the requester (in this case the private
key)). The MMA is also possible against [CMS]. Mail list agents are
the most likely CMS implementations to be targets for the MMA, since
mail list agents are automated servers that automatically respond to
a large number of messages. This document describes a strategy for
resisting such attacks.
2. Overview of PKCS-1
The first stage in RSA encryption is to map the message to be
encrypted (in CMS a symmetric content-encryption key (CEK)) into an
integer the same length as (but numerically less than) the RSA
modulus of the recipient's public key (typically somewhere between
512 and 2048 bits). PKCS-1 describes the most common procedure for
this transformation.
We start with an "encryption block" of the same length as the
modulus. The rightmost bytes of the block are set to the message to
be encrypted. The first two bytes are a zero byte and a "block type"
byte. For encryption the block type is 2. The remaining bytes are
used as padding. The padding is constructed by generating a series
of non-zero random bytes. The last padding byte is zero, which
allows the padding to be distinguished from the message.
+---+---+----------------------+---+---------------------+
| 0 | 2 | Nonzero random bytes | 0 | Message |
+---+---+----------------------+---+---------------------+
Once the block has been formatted, the sender must then convert the
block into an integer. This is done by treating the block as an
integer in big-endian form. Thus, the resulting number is less than
the modulus (because the first byte is zero), but within a factor of
2^16 (because the second byte is 2).
In CMS, the message is always a randomly generated symmetric
content-encryption key (CEK). Depending on the cipher being used it
might be anywhere from 8 to 32 bytes.
There must be at least 8 bytes of non-zero padding. The padding
prevents an attacker from verifying guesses about the encrypted
message. Imagine that the attacker wishes to determine whether or
not two RSA-encrypted keys are the same. Because there are at least
255^8 (about 2^64) different padding values with high probability two
encryptions of the same CEK will be different. The padding also
prevents the attacker from verifying guessed CEKs by trial-encrypting
them with the recipient's RSA key since he must try each potential
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RFC 3218 Preventing the Million Message Attack on CMS January 2002
pad for every guess. Note that a lower cost attack would be to
exhaustively search the CEK space by trial-decrypting the content and
examining the plaintext to see if it appears reasonable.
2.1. The Million Message Attack
The purpose of the Million Message Attack (MMA) is to recover a
single plaintext (formatted block) given the ciphertext (encrypted
block). The attacker first captures the ciphertext in transit and
then uses the recipient as an oracle to recover the plaintext by
sending transformed versions of the ciphertext and observing the
recipient's response.
Call the ciphertext C. The attacker then generates a series of
integers S and computes C'=C*(S^e) mod n. Upon decryption, C'
produces a corresponding plaintext M'. Most values of M' will appear
to be garbage but some values of M' (about one in 2^16) will have the
correct first two bytes 00 02 and thus appear to be properly PKCS-1
formatted. The attack proceeds by finding a sequence of values S
such that the resulting M' is properly PKCS-1 formatted. This
information can be used to discover M. Operationally, this attack
usually requires about 2^20 messages and responses. Details can be
found in [MMA].
2.2. Applicability
Since the MMA requires so many messages, it must be mounted against a
victim who is willing to process a large number of messages. In
practice, no human is willing to read this many messages and so the
MMA can only be mounted against an automated victim.
The MMA also requires that the attacker be able to distinguish cases
where M' was PKCS-1 formatted from cases where it was not. In the
case of CMS the attacker will be sending CMS messages with C'
replacing the wrapped CEK. Thus, there are five possibilities:
1. M' is improperly formatted.
2. M' is properly formatted but the CEK is prima facie bogus (wrong
length, etc.)
3. M' is properly formatted and the CEK appears OK. A signature or
MAC is present so integrity checking fails.
4. M' is properly formatted and no integrity check is applied. In
this case there is some possibility (approximately 1/32) that the
CBC padding block will verify properly. (The actual probability
depends highly on the receiving implementation. See "Note on
Block Cipher Padding" below). The message will appear OK at the
CMS level but will be bogus at the application level.
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RFC 3218 Preventing the Million Message Attack on CMS January 2002
5. M' is properly formatted and the resulting CEK is correct. This
is extremely improbable but not impossible.
The MMA requires the attacker to be able to distinguish case 1 from
cases 2-4. (He can always distinguish case 5, of course). This
might happen if the victim returned different errors for each case.
The attacker might also be able to distinguish these cases based on
timing -- decrypting the message and verifying the signature takes
some time. If the victim responds uniformly to all four errors then
no attack is possible.
2.2.1. Note on Block Cipher Padding
[CMS] specifies a particular kind of block cipher padding in which
the final cipher block is padded with bytes containing the length of
the padding. For instance, a 5-byte block would be padded with three
bytes of value 03, as in:
XX XX XX XX XX 03 03 03
[CMS] does not specify how this padding is to be removed but merely
observes that it is unambiguous. An implementation might simply get
the value of the final byte and truncate appropriately or might
verify that all the padding bytes are correct. If the receiver
simply truncates then the probability that a random block will appear
to be properly padded is roughly 1/32. If the receiver checks all
the padding bytes, then the probability is 1/256 + (1/256^2) + ...
(roughly 1/255).
2.3. Countermeasures2.3.1. Careful Checking
Even without countermeasures, sufficiently careful checking can go
quite a long way to mitigating the success of the MMA. If the
receiving implementation also checks the length of the CEK and the
parity bits (if available) AND responds identically to all such
errors, the chances of a given M' being properly formatted are
substantially decreased. This increases the number of probe messages
required to recover M. However, this sort of checking only increases
the workfactor and does not eliminate the attack entirely because
some messages will still be properly formatted up to the point of
keylength. However, the combination of all three kinds of checking
(padding, length, parity bits) increases the number of messages to
the point where the attack is impractical.
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RFC 3218 Preventing the Million Message Attack on CMS January 20022.3.2. Random Filling
The simplest countermeasure is to treat misformatted messages as if
they were properly PKCS-1 formatted. When the victim detects an
improperly formatted message, instead of returning an error he
substitutes a randomly generated message. In CMS, since the message
is always a wrapped content-encryption key (CEK) the victim should
simply substitute a randomly generated CEK of appropriate length and
continue. Eventually this will result in a decryption or signature
verification error but this is exactly what would have happened if M'
happened to be properly formatted but contained an incorrect CEK.
Note that this approach also prevents the attacker from
distinguishing various failure cases via timing since all failures
return roughly the same timing behavior. (The time required to
generate the random-padding is negligible in almost all cases. If an
implementation has a very slow PRNG it can generate random padding
for every message and simply discard it if the CEK decrypts
correctly).
In a layered implementation it's quite possible that the PKCS-1 check
occurs at a point in the code where the length of the expected CEK is
not known. In that case the implementation must ensure that bad
PKCS-1 padding and ok-looking PKCS-1 padding with an incorrect length
CEK behave the same. An easy way to do this is to also randomize
CEKs that are of the wrong length or otherwise improperly formatted
when they are processed at the layer that knows the length.
Note: It is a mistake to use a fixed CEK because the attacker could
then produce a CMS message encrypted with that CEK. This message
would decrypt properly (i.e. appear to be a completely valid CMS
application to the receiver), thus allowing the attacker to determine
that the PKCS-1 formatting was incorrect. In fact, the new CEK
should be cryptographically random, thus preventing the attacker from
guessing the next "random" CEK to be used.
2.3.3. OAEP
Optimal Asymmetric Encryption Padding (OAEP) [OAEP, PKCS-1-v2] is
another technique for padding a message into an RSA encryption block.
Implementations using OAEP are not susceptible to the MMA. However,
OAEP is incompatible with PKCS-1. Implementations of S/MIME and CMS
must therefore continue to use PKCS-1 for the foreseeable future if
they wish to communicate with current widely deployed
implementations. OAEP is being specified for use with AES keys in
CMS so this provides an upgrade path to OAEP.
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RFC 3218 Preventing the Million Message Attack on CMS January 20026. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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